Wavelength division multiplexing (WDM) is a technique employed in high capacity optical transmission networks to increase their information carrying capacity. WDM systems transmit a plurality of channels, each at a different wavelength, over a single optical fiber. Typically, a WDM system comprises N optical transmitters for optically transmitting N channels of information, a wavelength multiplexer for combining the N channels on one optical fiber, optical amplifiers connected in series by optical fiber cable, a wavelength demultiplexer for separating the optical signal into N channels, and N receivers for detecting the information carried by the N channels. In network configurations having an add-drop multiplexer, the multiplexer is located at a position along the path of the serially connected amplifiers. The add-drop multiplexer is capable of extracting a channel from the network for directing it to an alternative receiver and may also insert into the network a new channel transmitted at the same wavelength as the extracted channel.
Performance of WDM systems is related to the optical signal to noise ratio (OSNR) at the receiver. Due to wavelength dependent gain profiles, noise profiles and saturation characteristics of the optical amplifiers and wavelength dependent gain of other components in the optical path, channel OSNR values at the receiver will be unequal for equal transmitter optical power levels.
In U.S. Pat. No. 5,225,922 Chraplyvy et al disclose a method of equalizing channel performance in point-to-point WDM systems using the optical power level of each channel transmitter and the OSNR measured at each channel receiver. New transmitter optical power levels for each channel are calculated from the optical power level and OSNR values using a specified formula. The channel transmitters are then set to the new optical power levels. This process is repeated until the difference between the channel OSNRs is within a predetermined range. Although this method can equalize the OSNR at the receivers within a few iterations, it requires the use of expensive instruments such as an optical spectrum analyzer to measure the OSNRs at the receivers. A further disadvantage of this method is that it is not easily adaptable to more complex optical network configurations such as those having an add-drop multiplexer.
It appears that it would be advantageous to provide a less expensive and more flexible method of equalizing channel performance in WDM systems that is capable of equalizing performance in more complex networks such as those having an add-drop multiplexer.